Low-loss Fan-in/Fan-out Device for Coupled four-core Fiber Using a Femtosecond Laser Direct Inscription Technique

We demonstrate low-loss fan-in/fan-out device for $19 \mu \mathrm{~m}$-core-pitch coupled four-core fiber using a femtosecond laser direct inscription technique, achieving an average insertion loss of 0.49 dB before packaging and 0.92 dB after packaging.